As performance of semiconductor devices such as transistors, for example, silicon-germanium (SiGe) heterojunction bipolar transistors (HBT) continues to excel and clock speed or bandwidth of the transistors passes for example a few tens of GHz, demand on electric current supply for the transistors is generally expected to exceed the present DC current carrying capability of back-end-of-line (BEOL) metal interconnects, if the transistors are expected to continue achieving their desired and in certain instances optimum performance. For example, with the latest 200-GHz speed SiGe HBT devices that employ latest copper (Cu) wiring technology, demand on electric current supply may go as high as 6.5 mA/μm2 in density inside the related BEOL metal interconnects.
In the meantime, in order for transistors in particular HBT devices to operate reliably over their entire lifetime, under an ambient temperature of potentially as high as around 125 degree Celsius, the maximum current that present BEOL metal interconnects may be able to provide support or tolerate is estimated around 2 to 4 mA/μm2, depending on whether redundant stud contacts are used or not.
Obviously, if the HBT devices are routinely operated at close to a 6.5 mA/μm2 current density level in order to achieve their best possible performance, this over-the-limit high current density level may affect reliability of the HBT devices and ultimately cause shortened lifetime of the devices. It is evident that present BEOL metal interconnects or metal wirings are no longer capable of providing adequate support for the increasing demand of high current density (with device self-heating taken into account) that are necessary for proper operation of both recent and future generations of HBT devices.
Over the time, through carefully analysis of this device reliability issue which appears arising from the use of high current density, it becomes apparent to the applicants that one possible cause for this reliability concern may be attributed to an electro-migration phenomenon often observed in the metal interconnects. Electro-migration (EM) is known as a metal atom diffusion process in the presence of electron wind under DC bias condition. In the case of transistors being used under high current density, the electro-migration may induce void of a metal contact or wire at the cathode end, potentially causing an electrical open, and/or induce extrusion of a similar metal contact or wire at the anode end, potentially causing an electrical short. EM is also known as a thermally activated process that normally gets accelerated at elevated temperatures. In the case of a transistor, elevated temperature may be caused by, for example, device self-heating under even normal operating conditions.
FIG. 1A and FIG. 1B are demonstrative illustrations of simulated junction temperatures of different bipolar transistors and simulated limits of allowable electric current that may be supported by their related metal interconnects. For example, FIG. 1A illustrates different junction temperatures of transistors with different junction areas (101, 102, 103), at different power density levels of current. It is apparent that the junction temperature increases almost linearly with the increase in power density as well as with the increase in junction area (101-103). The dramatic increase in temperature is indicative that EM related reliability of metal interconnects or metal wirings shall be a concern for most circuitry designers. The level of sensitivity of EM related device performance degradation versus junction temperature of the device is illustrated in FIG. 1B by virtue of changes in DC current limit. For example, EM related device degradation may increase by a factor of five (5) by a temperature change of merely 25 degrees, from 100 C to 125 C, because the temperature change causes an approximately 80 percent decrease in DC current limit in the metal interconnects.
FIGS. 2A and 2B are demonstrative illustrations of interfaces at near collector contact and emitter contact of a typical HBT device with and without EM damages as is known in the art. For example, FIG. 2A is an illustrative showing of a HBT 200, together with its top view, before any EM damage occurs. HBT 200 may be supplied with an electric current which flows from emitter wire 201 to collector wire 202. After being operated for a certain period of time and in particular when the operation being under elevated temperatures, EM damages may start to occur in the emitter and collector contact regions of HBT 200. In particular, the EM damages may be around interface areas of between metal level one (M1), such as emitter wire 201 and collector wire 202, and contact areas (CA) of HBT device 200 as these areas are most susceptible to EM damage. For example, FIG. 2B illustrates that voiding occurs at the collector contact region (292) and extrusion occurs at the emitter contact region (291), both near the contact area (CA) of HBT device 200.